Pipe connection

ABSTRACT

Disclosed is a threaded pipe connection. The connection includes tubular externally threaded male members and a tubular internally threaded female coupling threadedly engaged with the male members. The male members include a continuous thread having a first preferably straight portion and a second tapered portion. The thread includes a form having stabbing and load blanks and an axially extending crest and root. Throughout the thread, lead as measured along the load flanks is constant and the length of the crest and depth of the load flanks are constant.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to threaded connections fortubular goods, and more particularly to a threaded connection for use ontubular goods of bimetallic construction as used in the production ofoil and gas.

B. Description of the Prior Art

It has become increasingly necessary to produce oil and gas fromreservoirs containing significant amounts of H₂ S and/or CO₂. Seriouscorrosion and embrittlement problems can occur when conventional carbonsteel tubular goods are used in such environments. Corrosion inhibitorshave been used, but these are costly and not always effective. Theindustry has considered using tubulars made entirely of stainless steel,nickel alloys, or even titanium alloys. However, such materials, whilegenerally having high performance characteristics, are very expensiveand may require special handling techniques.

The most satisfactory solution developed so far involves the use ofbimetallic tubes, which include a high strength outer pipe or shell withan inner liner or pipe of a corrosion resistant material. Such tubesoffer the advantages of having load-bearing and rugged-handlingcharacteristics of conventional steel tubes with the corrosionresistance of high-alloy materials.

Since oil field tubular goods come in discrete lengths, it is necessaryto connect them together to form strings. The connections must belikewise protected against corrosion or embrittlement. There have beenmanufactured for use with bimetallic pipe couplings of solid corrosionresistant material, such as stainless steel. However, such stainlesssteel couplings lack the strength and rugged handling characteristics ofordinary couplings. There have been developed for use with bimetallicpipe threaded connections of ordinary steel but with means forprotecting the connection material. Examples of such connections aredisclosed in U.S. Pat. Nos. 3,307,860; 3,336,054; 4,026,583; and4,373,750.

In addition to being able to withstand the corrosive effects of sour gasenvironments, pipes and their connections must be able to withstandservice in deep high-pressure wells. Wells in excess of 15,000 feet indepth and 20,000 pounds per square inch in pressure are not uncommon.Accordingly, pipes and connections must have high tensile strength andthe connections must provide effective leak proof seals. Additionally,oil field tubular goods are often subjected to rough treatment when theyare connected together on site. The threaded connections must beresistant to galling, jamming, cross threading, and mechanical damage tosealing surfaces.

It is therefore an object of the present invention to connect togetherbimetallic pipe in such way that the internal structure presents acontinuous metalurgical barrier while the external coupling maintainsthe mechanical properties of the pipe. It is a further object of thepresent invention to provide a threaded connection for use in sour gasenvironments that has a tensile strength approaching that of the pipesbeing interconnected. It is a further object of the present invention toprovide a threaded connection for use in sour gas environments that issubstantially leak proof at all operating pressure. It is a furtherobject of the present invention to provide a threaded connection that isresistant to galling, jamming, cross threading, and mechanical damage.

SUMMARY OF THE INVENTION

The tubular connection of the present invention includes first andsecond externally threaded male members and an internally threadedcoupling threadedly engaging the males members. Each male memberincludes an outer pipe and an inner pipe or liner formed of a corrosionresistant metal. The coupling is made of the same material as the outerpipes of the male members, but does not present itself to the inside ofthe pipe.

The threaded interconnection between each male member and the couplingis provided by a continuous helical thread having a first straight,untapered, portion and a second tapered portion. The thread has a formincluding stabbing and load flanks and an axially extending crest androot. Throughout the thread, lead as measured along the load flanks isconstant and the length of the crest and the depth of the load flank areconstant.

The ends of the male members are cut so as to form a metal-to-metalsubstantially impenetrable seal across the inside of the connection. Inone embodiment, the ends of the male members abut each other at a sealrim formed on the inner periphery of the male member. In a secondembodiment, the ends of the male members abut and seal on a tubularmetal ring manufactured of the same material as the metallic tube liner.When a preload is applied to male members, the end of each male memberseats itself against the other end or the metallic seat, due tocontrolled plastic deformation, and forms the primary seal for theconnection.

The coupling includes internal of the threads a pair of opposedoutwardly directed frusto-conical sealing surfaces and each male memberincludes an external frusto-conical sealing surface sealingly mateablewith the coupling. The frusto-conical sealing surfaces provide asecondary seal that is self-energizing in that as pressure inside thepipe increases, the metal is forced together for an improved seal. Thefrusto-conical sealing surfaces on the male members are spaced axially asubstantial distance apart from the threaded portion. Such spacingserves as a pilot during makeup which prevents or reduces the likelihoodof cross threading. Additionally, the spacing assists in theself-energizing feature of the secondary seal and causes a bendingmoment which enhances the primary seal. As weight or tensile force isapplied against the primary seal, the initial preload preserves theseal. However, should excessive weight be applied and the primary sealbreak, the secondary seal remains valid and will, in fact, be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a preferred embodiment of the tubularjoint of the present invention.

FIG. 2 is an enlarged cross section view showing details of the sealarrangement in the embodiment of FIG. 1.

FIG. 3 is a sectional view showing details of the seal area in analternative embodiment of the present invention.

FIGS. 4A and 4B are schematic views showing the method of threading themale member of the present invention.

FIGS. 5A and 5B are schematic views showing the method of threading thecoupling of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and first to FIG. 1, the tubular joint ofthe present invention is designated by the numeral 11. Joint 11includes, generally, a pair of externally threaded male members or pipes13a and 13b and an internally threaded coupling 15. Male members 13a and13b are substantially similar to each other and each includes an outerpipe 17 of ordinary steel and an inner pipe or liner 19 of corrosionresistant metal, for example, stainless steel.

Each male member 13a and 13b includes a continuous thread designatedgenerally by the numeral 21. Thread 21 includes a first, nontaperedportion 23 and a second tapered portion 25. Thread 21 includes agenerally trapezoidal thread from which includes a stabbing flank 27, aload flank 29, an axially extending crest 31, and axially extending root33. In the preferred embodiment, load flank 29 is reverse angled orhooked so as to enhance the tensile strength of the joint.

Throughout the length of thread 21, in both first nontapered portion 23and second tapered portion 25, thread lead, as measured along the loadflank parallel to the axis of the male member, is constant.Additionally, the axial length of crest 31 and the depth of the threadas measured from crest 31 to root 33 across load flank 29 are constant.

Coupling 15 is formed of tubular metal stock of the same material asouter pipe 17. Coupling 15 includes threaded portions which complimentand interfit with the dual tapered threaded portions, including thread21 of the male members.

Male members 13a and 13b each includes an external frusto-conicalsealing surface 35, which is separated from thread 21 by a cylindricalportion 37. Coupling 15 includes a pair of internal frusto-conicalsealing surfaces, including a surface 39 which mates and seals withsurface 35. Male member frusto-conical sealing surface 35 andcylindrical portion 37 also serve as pilot for guiding or stabbing themale members into coupling 15. The substantial length of cylindricalportion 37 coacts with the generally cylindrical surface formed by thecrests of the straight portion of the internal threads of coupling 15thereby to align the male member within the coupling to prevent crossthreading. Also, because of the dual taper of thread 21, firstnontapered thread 23 stabs into the tapered portion of coupling 15,thereby to reduce the likelihood of jamming as sometimes occurs when atapered pin is stabbed into a box of like taper. Additionally, the taperof second tapered portion 25 of thread 21 is preferably substantiallygreater than the standard 3/4 inch per foot on diameter. The steep taperenables the joint to go from hand tight to power tight very quickly andpreferably in less than one turn, thereby to reduce the likelihood ofgalling.

In the embodiment of FIG. 1, joint 11 includes a generally tubularwasher 41, which is carried within coupling 15 between the ends of malemembers 13a and 13b. Washer 41 is preferably formed of the samecorrosion resistant metal as that which forms inner pipe or liner 19.Washer 41 forms metal-to-metal seals with liners 19 and 19b at 43a and43b, respectively.

Referring now to FIG. 2, there is shown an enlargement of the seal areaof the embodiment of the invention shown in FIG. 1. Washer 41 has agenerally trapezoidal cross-section and has an inside diameter slightlysmaller than the inside diameters of male members 13a and 13b. Washer 41has a pair of sloping faces 45a and 45b and generally flat sealing rims47a and 47b about its inner periphery. Liners 19 and 19a of male members13a and 13b, respectively, define generally flat sealing rims 49a and49b, respectively which cooperate with rims 47a and 47b, respectively,to form metal-to-metal seals. The radially outer portions of the ends ofmale members 13a and 13b define frusto-conical faces 51a and 51b,respectively. Faces 51a and 51b extend radially forward of rims 49a and49b, thereby to protect rims 49a and 49b from mechanical damage. Theangles of faces 51a-b and 45a-b differ so as to define therebetween voidspaces 53a and 53b when joint 11 is made up hand tight. However, whenjoint 11 is made up power tight, voids 53a and 53b are lost tocontrolled plastic deformation.

Referring now to FIG. 3, there is shown an alternative embodiment of thepresent invention wherein male members 113a and 113b abut and seal oneach other. Liners 119 and 119a define sealing rims 149a and 149brespectively. Male members 113a and 113b include faces 151a and 151brespectively which define therebetween a void 153. In the embodiment ofFIG. 3, face 151b slopes axially forward of rim 149b and face 151aslopes axially rearward of rim 149a. In the embodiment of FIG. 3, malemember 113a would be inserted into coupling 115 at the manufacturingfacility so that coupling 115 would provide protection to rim 149a.

Referring now to FIGS. 4 and 5 there is shown generally the method bywhich the dual tapered thread of the present invention is cut. Referringfirst to FIGS. 4a and 4b, which show the method of cutting an externalthread, a thread cutting insert 61 is provided. Thread cutting insert 61includes a leading edge 63 for cutting the stabbing flank 27 a bottomedge 65 for cutting the root 33, a trailing edge 67 for cutting loadflank 29, and a topping edge 69 for cutting crest 31. To cut thread 21,the tubular work piece is rotated at a constant speed of rotation aboutits axis and thread cutting insert 61 is moved at a substantiallyconstant linear speed in the direction toward and parallel to the axisof the tubular work piece initially to cut first nontapered portion 23.When thread cutting insert 61 reaches the end of first nontaperedportion 23, which end is designated by the numeral 24, thread cuttinginsert 61 continues to move at the same constant speed parallel to theaxis of the tubular work piece, but additionally begins to move in adirection perpendicular to the axis of the work piece thereby to formsecond tapered portion 25.

Since throughout the cutting of thread 21, the work piece rotates at aconstant speed of rotation and thread cutting insert 61 moves at aconstant linear speed parallel to the axis of the work piece, leadmeasured along the load flanks parallel to the axis of the work pieceremains constant. However, it will be observed that the crests in secondtapered portion 25, as for example crest 26, are narrower than thecrests in first nontapered portion 23. The difference between crestlength in the straight and tapered portions causes the threaded profileto be nonuniform throughout the thread and may result in flank to flankinterference when the male and female members are connected together.Accordingly, the method of threading the joint of the present inventionincludes the further step of shortening the crests in the firstnontapered portion of the thread. As shown in FIG. 4b, in a final pass,thread cutting insert 61 is displaced in the direction toward the workpiece parallel to the axis thereof a distance equal to the differencebetween the crests in straight portion 23 and tapered portion 25. Withthread cutting insert 61 so displaced, the work piece is rotated at thesame constant speed of rotation and thread cutting insert 61 is movedaxially along the work piece. However, when thread cutting insert 61reaches the last straight thread, thread cutting tool 61 is pulledrapidly away from the work piece at an angle greater than the taperedportion so as not to affect the tapered threads.

FIGS. 5a and 5b depict schematically the cutting of the threads andcoupling 15. A thread cutting insert 71, which is similar to threadcutting insert 61 is provided. In cutting the thread, the tubular workpiece is rotated at a constant speed of rotation and thread cuttinginsert 71 is moved at a constant speed in the direction parallel to theaxis of the work piece and at a constant slower speed in the directionperpendicular to the axis of the work piece to cut the tapered portion73 when thread cutting insert 71 reaches the first thread 74 of thestraight portion 75, the speed component perpendicular to the axis ofthe work piece is stopped but the component parallel to the componentcontinues and thread cutting insert 71 cuts the straight portion 75.

Again, it will be observed that the crests in straight portions 75 arelonger than the crests in tapered portion 73. Consequently, as shown inFIG. 5b, the method of threading coupling 15 includes the step ofshortening the crest in straight portion 75. In the crest shorteningstep, thread cutting insert 71 is initially displaced radially outwardlyto correspond to the diameter of the straight portion 75 and axially adistance equal to the difference between the lengths of the crest andstraight portion 75 and tapered portion 73. With the work piececontinuing to rotate at the same constant speed of rotation, threadcutting insert 71 is moved in the direction parallel to the axis of thework piece at the same constant linear speed.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forthtogether with other advantages which are inherent to the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed with reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof is to be understood that all mattersherein set forth as shown in the accompanying drawings are to beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A tubular joint, which comprises:a tubularexternally threaded male member having a continuous helical threadhaving a first threaded portion with a flat angle of taper with respectto the axis of said male member and a second threaded portion with asecond angle of taper with respect to the axis of said male member, saidsecond angle of taper being greater than first angle of taper, saidhelical thread having a form including stabbing and load flanks and anaxially extending crest and root, wherein throughout said thread, leadas measured along the load flanks is constant and the length of saidcrest and the depth of said load flank are constant and the length ofthe root in said first threaded portion is longer than the length of theroot in said second threaded portion; and a tubular internally threadedfemale member having a continuous helical thread mateable with saidhelical thread of said male member, said helical thread of said malemember having a first threaded portion with a first angle of taper withrespect to the axis of said female member substantially equal to saidfirst angle of taper of said male member, and a second angle of taperwith respect to said axis of said female member substantially equal tosaid second angle of taper of said male member, said female memberhelical thread having a form including stabbing and load flanks and anaxially extending crest and root, wherein throughout said female helicalthread, lead as measured along the load flanks is constant and thelength of said crest and the depth of said load flank are constant andthe length of the root in said first threaded portion is longer than theroot in said second threaded portion.
 2. The tubular joint as claimed inclaim 1, wherein said first angles of taper of said first threadedportions of said male and female members are substantially equal tozero.
 3. The tubular joint as claimed in claim 1, wherein:said femalemember includes an internal frusto-conical sealing surface interior ofsaid helical thread; said male member includes an externalfrusto-conical sealing surface sealingly mateable with said sealingsurface of said female member.
 4. The tubular joint as claimed in claim3, wherein:said male member includes an end having a sealing rim aboutthe inner periphery of said male member and a front face extendingradially outwardly from said sealing rim; and said female memberincludes shoulder means having a sealing rim for sealingly engaging saidsealing rim of said male member and including a shoulder means frontface extending radially outwardly from said sealing rim of said femalemember, wherein at initial contact of said sealing rims said front facesare axially spaced apart.
 5. The tubular joint as claimed in claim 4,wherein said front face of said male member end slopes axially from saidsealing rim toward said female member shoulder means.
 6. The tubularjoint as claimed in claim 4, wherein:said male member includes inner andouter pipes, said inner pipe being formed of corrosion resistant metal;and said female member shoulder means includes an inner portionincluding said female member shoulder sealing rim formed of corrosionresistant metal.
 7. The tubular joint as claimed in claim 6, whereinsaid female member shoulder includes a tubular corrosion resistant metalwasher contained within said female member.
 8. The tubular joint asclaimed in claim 4, wherein:said female member shoulder means includes asecond tubular male member threadedly engaged with said female member.9. The tubular joint as claimed in claim 8, wherein:said second tubularmale member includes a continuous helical external thread having a firstthreaded portion with a first angle of taper with respect to the axis ofsaid second male member and a second threaded portion with a secondangle of taper with respect to said axis of said second male member,said second angle of taper being greater than said first angle of taper,said second male member helical thread having a thread form includingstabbing and load flanks and an axially extending crest and root,wherein throughout said second member helical thread, lead as measuredalong the load flanks is constant and the length of said crest and thedepth of said load flank are constant and the length of the root in saidfirst threaded portion is longer than the length of the root in saidsecond threaded portion; and said female member has a second internalcontinuous helical thread mateable with said second male member helicalthread, said female member second helical thread having a first threadedportion with a first angle of taper with respect to said axis of saidfemale member substantially equal to said second male member first angleof taper, and a second threaded portion with a second angle of taperwith respect to said axis of said female member substantially equal tosaid second male member second angle of taper, said female memberhelical thread having a form including stabbing and load flanks and anaxially extending crest and root, wherein throughout said female membersecond thread, lead as measured along the load flanks is constant andthe length of said crest and the depth of said load flank are constantand the length of the root in said first threaded portion is longer thanthe length of the root in said second threaded portion.
 10. The tubularjoint as claimed in claim 9, wherein said first angles of taper of thefirst male member and the second male member are substantially equal andthe second angles of taper of said first and second male members areequal.
 11. The tubular joint as claimed in claim 10, wherein said firstangles of taper are substantially zero.
 12. The tubular joint as claimedin claim 9, wherein said male members each include inner and outer pipe,said inner pipes being formed of a corrosion resistant metal.
 13. Thetubular joint as claimed in claim 1, wherein said load flanks of saidmale and female member threads are reverse angled.
 14. The tubular jointas claimed in claim 1, wherein the angle of said stabbing flank of saidmale member thread with respect to said male member axis is greater thanthe male member second angle of taper with respect to said male memberaxis.
 15. A tubular joint, which comprises:first and second externallythreaded male members, said male members each including an outer pipeand an inner pipe, said inner pipe being formed of a corrosion resistantmetal, each of said male members including an unthreaded inner endhaving an external frusto-conical sealing surface and a sealing rimabout the inner periphery of said inner pipe and having a front faceextending radially outwardly from said sealing rim; and an internallythreaded coupling threadedly engaging said male members, said couplingincluding a pair of oppositely outwardly directed fursto-conical sealingsurfaces therein matingly sealing engaging said fursto-conical sealingsurfaces of said male members, wherein said sealing rims of said malemembers, sealingly abut and seal with each other, and when said sealingrims initially abut, said front faces are spaced apart.
 16. A tubularjoint, which comprises:first and second externally threaded malemembers, said male members each including an outer pipe and an innerpipe, said inner pipe being formed of a corrosion resistant metal, eachof said male members including an unthreaded inner end having anexternal fursto-conical sealing surface and a sealing rim about theinner periphery of said inner pipe and having a front face extendingradially outwardly from said sealing rim; an internally threadedcoupling threadedly engaging said male members, said coupling includinga pair of oppositely outwardly directed frusto-conical sealing surfacetherein matingly sealing engaging said frusto-conical sealing surface ofsaid male members; and a tubular washer of corrosion resistant metalwithin said coupling between the ends of said male members, the ends ofsaid washer including sealing rims adjacent the inner periphery thereofwith end faces extending radially outwardly therefrom, said sealing rimsof said male members sealingly engaging the sealing rims of said washer,wherein when said sealing rims of said male members and said washerinitially abut said front faces and end faces are spaced apart.